1. Field of the Invention
This invention relates to a collimator for a radiation detector, such as an X-ray camera or a nuclear radiation detecting scintillation camera used for imaging distributions of radioactive material in organs of the human body.
2. Description of the Prior Art
Radiation detectors, such as nuclear radiation detectors like scintillation cameras, are widely used to develop information for medical diagnostic purposes based on signals derived from a source of radiation. A well-known radiation detector of a type to which the present invention finds application is the Anger-type scintillation camera, the basic principles of which are explained in U.S. Pat. No. 3,011,057. The Anger camera is used to determine the distribution of a radiation emitting substance in a human body organ by analyzing the locations of scintillation events which occur on a crystal due to rays incident from the body organ. The Anger camera and other radiation detectors typically employ a radiation collimator between the radiation sensitive transducer (e.g. the crystal in the Anger camera) and the source or radiation.
The purpose of using a radiation collimator is to provide radiation transmissive passageways to ensure a mapping correspondence between respective elemental volumes of the radiation source (e.g. the body organ) and elemental volumes of the transducer (e.g. the Anger camera crystal). The most commonly used collimator is a multi-channel collimator comprising a number of collimating apertures separated from each other by walls or septa of a solid radiation opaque materal--most commonly lead.
It is well known that radiation collimator design involves basically the parameters of aperture size and shape, septal thickness, and aperture length. These are the parameters which determine the resolution and efficiency of the collimator for radiation (e.g. gamma rays) of a particular energy. In general, the septal thickness, which is the thickness of the walls separating adjacent collimating apertures, is chosen in accordance with the energies of the rays to be collimated so that the collimator will block the rays which enter the collimator at an angle and location such that they would otherwise traverse the wall between two apertures. Thus the septal thickness must be relatively large for collimators used with high energy radiation sources, but for low energy sources the septum or wall between the apertures may be quite thin. Indeed, it is desirable to employ only the septal thickness actually required for the radiation energy involved in order to avoid unnecessary loss of efficiency.
Many multi-channel collimators are made of cast lead. Pins of desired cross-sectional shape are placed into pilots or recesses of a casting basin to form a nest. Molten lead is then poured into the basin to flow into the spaces between adjacent pins and between the pin nest and the basin wall. After the lead is allowed to cool somewhat, the casting is removed from the basin and the pins are manually beaten or pulled out of the casting one at a time. The holes left in the casting by the removal of the pins form the collimator apertures. The hardened lead occupying the spaces left between the pins forms the collimator septa. A collimator formed by the casting process can have relatively evenly spaced and uniform apertures. However, the manual loading of pins into the casting basin and the manual removal of the pins from the cooled casting is both tedious and time-consuming. And, although the lead is cool relative to its temperature when molten at the time of pin removal, even though protective clothing is worn, there may still be a risk of burns when the pins are removed.
Other collimators are formed without the necessity of the described tedious pin setting and removal process by a cold extrusion and gluing method. Such collimators are built up from layers or "slices" of cold-shaped lead which are glued side-by-side into a sandwich collimator structure. U.S. Pat. Nos. 3,921,000 and 3,943,366 disclose examples of structures having strips of corrugated lead foil fastened together in rows to build up a collimator structure of the desired size. The manufacture of collimators using cold-extruded slices, like the corrugated strips disclosed in U.S. Pat. Nos. 3,921,000 and 3,943,366, however, presents severe tolerance problems. The corrugations must be extremely uniform from strip to strip or they will not match up at the surfaces which are to be mated and fastened together throughout the length of each strip. Furthermore, the use of lead as the corrugated material creates additional problems due to its very low tensile strength. The manufacture of collimators by this build-up process is especially troublesome in the manufacture of "diverging," "converging" and "multi-view" collimators.
The terms "diverging" and "converging" are defined viewing the collimator from the transducer side. A "diverging" collimator is one having channels focused at a point some distance away and arranged to diverge in the transducer, so that objects smaller than the transducer can be imaged in a magnified way. A "converging" collimator is one having channels arranged to converge in the transducer, so that objects larger than the transducer can be imaged. A "multi-view" collimator, such as disclosed in U.S. Pat. No. 4,181,839, includes sets of channels oriented so that a plurality of simultaneous views of the same object of diagnosis can be obtained.